This disclosure relates generally to the field of construction of wellbores through subsurface formations. More particularly the disclosure relates to methods for automatically evaluating mechanical and hydraulic conditions within a wellbore during drilling operations so as to reduce chances of a drilling tool assembly and/or drill string become stuck in a wellbore or inducing damage to any part of the drill string.
Drilling wellbores through subsurface formations includes suspending a “string” of drill pipe (“drill string”) from a drilling unit or similar lifting apparatus and operating a set of drilling tools and rotating a drill bit generally disposed at the bottom end of the drill string. The drill bit may be rotated by rotating the entire drill string from the surface and/or by operating a motor disposed in the set of drilling tool. The motor may be, for example, operated by the flow of drilling fluid (“mud”) through an interior passage in the drill string. The mud leaves the drill string through the bit and returns to the surface through an annular space between the drilled wellbore wall and the exterior of the drill string. The returning mud cools and lubricates the drill bit, lifts drill cuttings to the surface and provides hydrostatic pressure to mechanically stabilize the wellbore and prevent fluid under pressure disposed in certain permeable formations exposed to the wellbore from entering the wellbore. The mud may also include materials to create an impermeable barrier (“filter cake”) on exposed formations having a lower fluid pressure than the hydrostatic pressure of the mud in the annular space so that mud will not flow into such formations in any substantial amount.
It is known in the art to determine the condition of the wellbore with regard to removal of drill cuttings (“hole cleaning”) by torque and drag plots. There are modeling programs known in the art that may be used to calculate an expected torque to be applied to the drill string and an amount of axial force consumed by friction between the wellbore wall and the drill string in view of drill string configuration and properties of the mud. The expected torque and drag may be compared to the measured torque and drag to determine if the cuttings are being completely removed, it being presumed that increases in either torque and/or drag values are indicative of incomplete hole cleaning. As a matter of ordinary practice this is done infrequently and is done more often in well planning and not during actual wellbore drilling operations. If torque and drag plots are used for operational analysis, they are typically performed manually.
The same type of analysis is known in the art to be performed for stand-pipe pressure (the pressure of the mud at the point at which it enters the drill string) and ECD (equivalent circulating density), that is, expected values modeled prior to or during drilling operations are compared to measured values. All of the analysis systems known in the art review individual symptoms of incomplete hole cleaning in isolation and do not provide the wellbore operator with an integrated analysis of all factors related to hole cleaning that may be indicative of increased risk of the drill string becoming stuck in the wellbore.
There are also programs known in the art that measure certain parameters and calculate a risk of differential sticking (i.e., the drill string becoming stuck by wiping through the filter cake in a formation with substantially lower fluid pressure than the hydrostatic pressure in the wellbore). Such programs are manually operated and may be difficult for the drilling unit operator to observe increased stuck pipe risk until it is too late.